In the development of a prosthetic foot for amputees, the SACH (solid ankle, cushion heel) foot innovation was considered a benchmark in modern prosthesis for its comfort and energy conservation. The original SACH concept has been modified in the ensuing decades by providing various forms of articulated keels to increase energy return, as well as using modern polymer and expanded polymer materials to increase comfort. The purpose of a flexible, elastic keel is to store energy during the foot-flat and rollover portions of the walking movement, followed by releasing of energy from the keel during the toe-off motion.
Several deficiencies have been noted in these prosthetic feet, even though they have achieved user acceptance. One major problem is that many of the prosthetic foot constructions include precision machined components that are expensive to manufacture and contribute to a high unit cost. Moreover, the metal components, which are generally embedded in a cast polymer or expanded polymer body, are not compatible with the polymer material on a long-term basis. That is, the polymer or expanded polymer material is much more resilient and flexible than the metal (typically steel, aluminum, or brass), and the constant flexion of the prosthetic foot during use leads to delamination of the interface between the metal components and the polymer-based material. Thus the prostheses tended to wear out after a relatively short period.
More significantly, prior art prosthetic feet have been designed with a focus on longitudinal flexion for energy recovery in a forward walking motion on a level surface. However, these devices have poor performance in lateral flexion motion, since the need for such flexure was not recognized in the prior art. Lateral flexion is required for many actions, due to the fact that real-world activities take place on sidewalks or roadways that are uneven and not level, and that obstacles such as curbs, broken pavement, and the like are often encountered. Due to the fact that prior art prosthetic feet have not been able to undergo lateral flexion, there has been painful wear and tear on the amputees' stumps from simple daily activities as well as running, jumping, dancing and the like.